An improved display system has indicia which is readable in bright sunlight when a light source within the display system is energized. However, when the light source in the display system is not energized, the indicia is not readable, even in bright sunlight.
Air crew station signaling devices used in military and commercial aircraft, are frequently exposed to high ambient incident illumination (direct sunlight irradiation). These signaling devices must maintain a high brightness contrast characteristic when the signaling device is activated in order to enable indicia on the signaling device to be read. However, when the signaling device is inactive and is exposed to high ambient incident illumination, the indicia should not be readable in order to avoid the transmission of false signals to the crew of the aircraft.
Known signalling devices for the crew of an aircraft have a plurality of miniature incandescent lamps which provide illumination of indicia. Signal intensity, brightness, uniformity, and color with which the indicia are seen by observers, are the collective result of the arrangement of optical filters and other associated optical components. To achieve high brightness contrast characteristics suitable for a direct sunlight environment, high candle power, high wattage type miniature incandescent lamps have been used. These lamps have previously been employed in as many as four in a group, but not less than two, usually arranged parallel or symmetrically to the indicia.
The use of high energy miniature incandescent lamps to achieve high brightness contrast, presents some disadvantages that are detrimental to the operating characteristics of the signalling device. Some of the disadvantages include relatively high power consumption in order to obtain the necessary illumination. The high power consumption may result in excessive temperatures within the signalling device. The excessive temperatures may be so great as to cause the temperature of the signalling device to exceed touch temperature limits. The high power consumption may result in the incandescent lamps having a relatively short operating life. Thus, there is a need for a signalling device which is readable in direct sunlight without the thermal problems normally associated with four-lamp systems which have previously been used.